I found an easy way to downgrade Horizon Zero Dawn using HZDDowngrader. Here's a step-by-step guide:

1. Download HZDDowngrader:
   Download it from this link: https://github.com/PercentBoat4164/HZDDowngrader
   
2. Unzip the downloaded file.
   
3. Install required programs:
   Make sure you have these programs installed: unzip, wget, basename, dotnet, md5sum. If you don't have dotnet, install it with:

Open HorizonZeroDawnDowngradeScript.sh with an edit programs like notepadqq.

And edit the top line.

To where you have your game if its in the main folder like .steam, then it has to look like this


and save the file.

Now make the file execute
by right click on it and properties

Open the terminal and say CD (link to HZDDowngrader-1.1 Folder) and push enter.
now say

and push enter.

You will be prompted for Steam username and password as well as the 2FA code two times.

Then it will do the rest for you.

Just start the game, and it should work.